1. Field of the Invention
This invention relates to an electrolytic cell used for producing a nitrogen trifluoride gas by a molten salt electrolysis.
2. Description of Related Art
A nitrogen trifluoride gas is used as a dry etching agent for semiconductors and a cleaning gas for CVD apparatuses. Its demand for these uses has been recently increased. In such applications, a nitrogen trifluoride gas of high purity, in particular, the content of carbon tetrafluoride being low, should be used.
NF.sub.3 gas can be manufactured by various methods. Among them, a molten salt electrolysis gives good yield and is suitable for mass production as compared with other methods and therefore, is regarded as useful commercial processes. In particular, for the purpose of producing a highly pure NF.sub.3 gas containing only a small amount of CF.sub.4, the molten salt electrolysis method can produce NF.sub.3, at the lowest cost and thereby, the method is expected to be an advantageous method. In general, according to a process for producing NF.sub.3 gas by a molten salt electrolysis, exemplary suitable molten salt baths comprise acidic ammonium fluoride, NH.sub.4 F.HF systems derived from ammonium fluoride and hydrogen fluoride, or KF.NH.sub.4 F.HF systems produced by adding acidic potassium fluoride or potassium fluoride or potassium fluoride to the NH.sub.4 F.HF system.
However, investigations on electrolytic cells have been scarcely made upon scaling up the cells to an industrial scale in the production of NF.sub.3 gas by molten salt electrolysis. In particular, nothing has been reported as to the concrete structure of such scaled-up electrodes.
When electrolytic cells are scaled up, it is advantageous to suppress the increase in the cross sectional area of the electrolytic cell as far as possible and increase the height since this results in a small floor area necessary for the electrolytic cell and, in addition, the vaporization amount of HF in a molten salt becomes relatively small.
In the process of manufacturing NF.sub.3 gas, NF.sub.3 gas and nitrogen (N.sub.2) gas are generated at the anode while hydrogen (H.sub.2) gas is generated at the cathode. That is, so-called gas generating reactions occur at the both electrodes. When NF.sub.3 gas generated at anode is mixed with H, gas generated at cathode, there is a fear of explosion and therefore, it is necessary to effect a safety countermeasure so as not to cause explosion.
In order to prevent explosion, an electrolytic cell is provided with a partition plate for separating anode and cathode as illustrated in FIGS. 1 and 2.
In such an electrolytic cell having a partition plate, a current hardly flows from an anode to a cathode at a region where the partition plate separates an anode and a cathode, but can flow only at a region situated lower than the lower end of the partition plate.
For the purpose of inhibiting corrosion of the partition an electrode, it is usually preferable to use a fluororesin as the partition plate or to cover the partition plate with a fluororesin.
When a partition plate is made of or covered with a fluororesin, a current does not flow at all from an anode to a cathode at a region where both electrodes are separated by such a partition plate.
As a material for anode, a carbon (C) or nickel (Ni) electrode can be used, and a nickel electrode is preferably uses as an anode so as to obtain a highly pure gas containing less amount of CF.sub.4. However, when a nickel electrode is used, there is a drawback that nickel is slightly dissolved.
The present inventors used a nickel anode for a long time. A part of the dissolved nickel precipitated on the cathode, and while the electrolysis was carried out for a long period of time, the distance between the cathode and the partition plate gradually became small.
As a result, when the distance between the cathode and the partition plate is too small, H.sub.2 gas generated at cathode and NF.sub.3 gas generated at anode are mixed and there is a fear that a gas mixture within explosion limits is formed.
When bubbles of NF.sub.3 gas generated at the Ni electrode ware observed, it was found that many small bubbles were formed, and therefore, the bubbles could not rise directly upward along the electrode, but diffused obliquely upward.
The present inventors used the electrodes for a long period of time and found that the anode was getting shorter with the lapse of time and the current density at anode increased. As a result, the amount of NF.sub.3 gas generated per unit area of the Ni anode increased and the diffusion of the NF.sub.3 gas became more vigorous. As NF.sub.3 gas diffused more vigorously, NF.sub.3 gas generated at anode and H.sub.2 gas generated at cathode were mixed when the distance between the partition plate and the anode was too small, and as mentioned above, there was a fear that a gas mixture within the explosion limits was formed in the cathode region.
As mentioned above, in the case of the production of NF.sub.3 gas according to a method of a molten salt electrolysis, the distance between a partition plate separating an anode and a cathode and the anode and the distance between the partition plate and the cathode are very important from the standpoint of safety. However, investigation as the structure of electrolytic cell has not been substantially made, and in particular, there is not reported any concrete structure and configuration of electrodes and partition plates.
Further, when Ni electrodes are used, there is a disadvantage that the nickel is slightly dissolved in a electrolytic bath. When the present inventors used nickel electrodes for a long time, a part of the dissolved nickel deposited in the form of nickel fluoride at the bottom of an electrolytic cell, and while the electrolysis was carried out for a long period of time, the deposit piled on the bottom surface of the electrolytic cell. It was found that as the nickel fluoride deposited on the bottom surface of the electrolytic cell, the distance between the lower end of the electrode plate and the piled matter became small.
Therefore, when the distance between the lower end of electrode and the bottom surface of the electrolytic cell is too small, the lower end of an electrode which is nearer to the bottom surface than the other electrode begins first to be gradually buried in the nickel fluoride, and the portion of the electrode thus buried can not function as an electrode any more. As a result, the area of the electrode capable of functioning as an electrode is decreased and the current density increases resulting in rise of the voltage of electrolytic cell and poor yield. Consequently the short distance between the lower end of electrode and the bottom surface is not desirable.
In addition, when the depositing of the dissolved nickel proceeds further and both electrodes are buried in the deposit resulting in short circuit. Thus, in an extreme case, such a situation is very dangerous and explosion and a fire are caused.
It has been found that the distance between the lower end of electrode and the bottom surface of the electrolytic cell is an important problem concerning safety upon using electrolytic cells for a long period of time.
Further, the convection in an electrolytic bath in an electrolytic cell has been now found the be such that in an electrolytic bath a flow from the lower part to the upper part occurs at a region where gases near electrodes rise due to gases generated at both electrodes while the portion of the electrolytic bath having risen to the upper part reversely flows downward at a region apart from the electrodes, and this convection serves to remove Joulean heat generated between the two electrodes by electrolysis by external or internal cooling and thereby the temperature distribution in the electrolytic bath in the electrolytic cell can be kept substantially uniform.
Therefore, when the distance between the lower end of electrode and the bottom surface is to large, a convention due to gas generation is not caused in the portion of electrolytic bath near the bottom of the electrolytic cell because said portion is far from the lower end of electrode and neither is generated Joulean heat, and therefore, on the contrary, the temperature of the portion of electrolytic bath near the bottom surface is lowered too much resulting in change of the bath composition, and in an extreme case, there is a fear that said portion solidifies. Therefore, it is necessary to cool the portion of electrolytic bath near the upper part of the electrolytic cell while the lower part of the cell should be heated. It is a big problem that such complicated operation is required.
As mentioned above, upon producing NF.sub.3 gas according to a molten salt electrolysis, the distance between the lower end of each of anode and cathode and the bottom surface of the electrolytic cell has now been found very important for a stable operation. However, there has not been substantially made any investigation as to the structure of electrolytic cell and, in particular, there is not any report on the distance between the lower end of electrode and the bottom surface of the electrolytic cell.
Furthermore, the temperature of molten salt upon electrolysis according to a method of a molten salt electrolysis is most preferably 100.degree.-130.degree. C. since the operation is easy, the electroconductivity is good and, in addition, the electric current efficiency is excellent.
However, when the temperature of the molten slat is 100.degree.-130.degree. C. in the NH.sub.4 F-HF system, the NH.sub.4 F.HF (melting point of 126.degree. C.) evaporated due to the vapor pressure disadvantageously deposits at a portion where the temperature is lower than the electrolytic bath.
When the present inventors carried out a continuous electrolysis for a long period of time, it was observed that a part of the NH.sub.4 F-HF system evaporated deposited on a lid of the electrolytic cell and outlets for generated gases as NH.sub.4 F.HF, and the gas outlets were easily clogged.
Thus, the present inventors tried to use the electrolytic cell continuously for a long period of time while flowing a carrier gas so as to prevent clog of gas outlets, but it was found that NH.sub.4 F.HF deposited even on the inlet of the carrier gas and the inlet was also clogged. When carrier gas inlets and generated gas outlets are clogged as mentioned above, a pressure difference is formed between the anode chamber enclosed with partition plates and containing the gas generated at anode, NF.sub.3, and the cathode chamber enclosed with partition plates and containing the gas generated at cathode, H.sub.2, and thereby a liquid surface level difference is formed resulting in a cause of big trouble.
For example, when the outlet for the gas generated at anode is clogged, NF.sub.3 gas can not be exhausted from the anode chamber and the generation of NF.sub.3 gas continues and thereby the pressure in the anode chamber rises. As a result, the liquid surface in the anode chamber is pushed down while the liquid surface in the cathode chamber is pushed up. When the liquid surface in the anode chamber is pushed down to a level lower than the lower end of the partition plate, NF.sub.3 gas in the anode chamber enters the cathode chamber to form a gas mixture within explosion limits and thereby the gas mixture is liable to explode in the cathode chamber.
Once explosion occurs, a part of an electrolytic cell is destroyed and, in addition, hydrofluoric acid, a very corrosive chemical, is released and therefore, this probably results in a serious accident, and production of NF.sub.3 will be not possible any more.
When an outlet for the gas generated at anode is clogged in the anode chamber, a big accident as mentioned above occurs. When the clogging occurs in the cathode, the same accident also occurs. Therefore, clog of gas inlet and outlet is to be essentially avoided from the standpoint of safety.
However, these problems are not yet known well and any effective countermeasures have not yet been proposed.
In the molten salt electrolysis, NF.sub.3 gas and H.sub.2 gas generated at the electrodes rise along the respective electrodes in the electrolytic bath. Large amounts of the gases rising in the electrolytic bath are present at the upper part of the electrolytic bath and the current is interrupted by the gases so that the current flows with difficulty. As a result, there is formed a distribution of electric current density in the vertical direction of the electrodes such that the density is smaller at the upper part of the electrodes and larger at the lower part thereof. In an extreme case, electrolysis scarcely proceeds at the upper portion of the electrode (in the region situated lower than the partition plate).
In view of the foregoing, the above-mentioned method for scaling up the electrolytic cell comprising limiting the cross sectional area of the electrolytic cell as far as possible so as to reduce the floor area of the electrolytic cell and increasing the height can be used only in a limited range since the length of the electrode increases in the vertical direction correspondingly and the distribution of electric current density in the vertical direction becomes largely nonuniform and the electric current efficiency (the ratio of the electric power consumed for producing NF.sub.3 to the amount of electric power applied) is lowered.
Furthermore, when the vertical length of electrode is large, the distance between the lower end of the electrode and the partition plate is also automatically long, and therefore, the amount of diffusion of the gas generated by electrolysis increases correspondingly, and NF.sub.3 gas and H.sub.2 gas are easily mixed disadvantageously resulting in possible explosion.
In the mean time, when the raw material, molten salt, contains water in the production of NF.sub.3 gas by a molten salt electrolysis, it appears that the resulting fluorine reacts with water to form OF.sub.2 gas and H.sub.2 gas.
According to a literature, J. Massome, Chem. Ing. Techn., 41, 695 (1969), the mechanism for forming NF.sub.3 gas by molten salt electrolysis is as shown below. That is, fluorine formed at an anode according to the following formula 1) reacts with ammonium ions in a molten salt, and according to the following formula 2) NF.sub.3 gas is generated at the anode while H.sub.2 gas is generated at the cathode. EQU 6F.sup.- .fwdarw.6F+6e.sup.- 1) EQU 6F+NH.sub.4.sup.+ .fwdarw.NF.sub.3.sup..uparw. +4H.sup.30 +3F.sup.- 2)
However, according to the knowledge of the inventors, it is considered that when water is present in the molten salt, OF.sub.2 gas and H.sub.2 gas are formed according to the following formulas 3) and 4). And the resulting OF.sub.2 gas and H.sub.2 gas are contained in the generated NF.sub.3 gas. EQU 2F+H.sub.2 O.fwdarw.OF.sub.2 +2H.sup.+ 3) EQU 2F+H.sub.2 O.fwdarw.OF.sub.2 +H.sub.2.sup..uparw. 4)
The presumed reactions in formulas 3) and 4) above can be supported by the fact that the concentrations of both OF.sub.2 and H.sub.2 in a gas generated at an anode become low as the electrolysis time becomes long. When OF.sub.2 and H.sub.2 are mixed in the gas generated at an anode, there is a danger of explosion so that it is extremely undesirable.
However, NH.sub.4 F.HF molten salt is so hygroscopic that it is liable to absorb moisture in air at the stage of preparing the raw material. Therefore, for preparing NF.sub.3, a dehydration electrolysis is indispensable which is effected by flowing a current at a current density lower than that at a main electrolysis, and after completion of the dehydration electrolysis, a main electrolysis is subsequently carried out.
However, in this dehydration electrolysis there occur not only generation of NF.sub.3 gas at an anode by the above-mentioned reactions of formula 1) and formula 2), but also the reactions of formula 3) and formula 4). Therefore, there is a danger of explosion as well as mixing of H.sub.2 gas generated at a cathode with a gas generated at an anode due to the diffusion of H.sub.2 when an electrode is longitudinally long. The water content in the molten salt upon the dehydration electrolysis is rather more than that upon the main electrolysis so that possibility of explosion is stronger.
The above-mentioned electric current efficiency is usually 60-70% in the production of NF.sub.3 gas according to a molten salt electrolysis.